Arsenic-based life-form upends genetic model

Bacterium opens the door to a possible 'shadow biosphere'

WASHINGTON - All life on Earth, from microbes to humans, is based on a single genetic model that requires the element phosphorus as one of its six essential components.

But now researchers, led by a biochemist at Arizona State University, have uncovered a bacterium that has replaced one of those essential elements, phosphorus, with its toxic cousin, arsenic.

News of the discovery caused a commotion, including calls to NASA from the White House and Congress asking if a second line of earthly life has been found.

A NASA news conference Thursday and an article in the journal Science gave the answer: No, the discovery does not prove the existence of a so-called "second genesis" on Earth. But it does open the door to that possibility, and to the related existence of a theorized "shadow biosphere" on Earth - life evolved from a different common ancestor than all that we've known so far.

It also expands the scope of the search for life-forms on other planets.

"Our findings are a reminder that life-as-we-know-it could be much more flexible than we generally assume or can imagine," said Felisa Wolfe-Simon, the lead author of the study. She is a NASA Astrobiology Research Fellow and a member of the NASA Astrobiology Institute team at ASU.

"If something here on Earth can do something so unexpected - that breaks the unity of biochemistry - what else can life do that we haven't seen yet?"

The research, funded through NASA and conducted with samples from California's Mono Lake, found that some of the bacteria not only used arsenic to live, but had arsenic embedded in their DNA, RNA and other basic underpinnings.

When searching for life beyond Earth, scientists look for six essential chemical elements - carbon, oxygen, hydrogen, nitrogen, sulfur and phosphorus - that are known as the building blocks of life. Thursday's discovery suggests that scientists need to think differently about which elements to follow in their search for life, as organisms can have a chemical architecture different from what is understood to be possible.

"It challenges our assumptions," said Ariel Anbar, an ASU professor in the School of Earth and Space Exploration, co-author of the paper and director of the Astrobiology Institute team at ASU. "We have this certain idea of how life works. We have this box, and this kicks it outside the box."

Mary Voytek, senior scientist for NASA's program in astrobiology, said, "We don't know if the arsenic replaced phosphorus or if it was there from the very beginning - in which case it would strongly suggest the existence of a shadow biosphere."

Paul Davies, director of the Beyond Center at ASU and a co-author of the paper, had been thinking about that idea for a decade and had written a paper in 2005. So had University of Colorado, Boulder philosopher and astrobiologist Carol Cleland. Both asked why nobody was looking for life with different origins on Earth, and Cleland coined the phrase "shadow biosphere."

At a Beyond Center conference four years ago, Wolfe-Simon, then in her late 20s and a postdoctoral scientist at ASU, proposed a way to search for a possible shadow biosphere, and it involved Mono Lake and its arsenic. At the time, some scientists viewed it as a wild idea, but the proposal was memorable because it was concrete, Davies said.

"The credit goes to Felisa," Davies said. "I'm just sort of the father figure and well wisher. I was intrigued by the idea and felt she would pursue it. NASA came to rescue to fund it."

Wolfe-Simon selected Mono Lake as a work site because the lake receives run-off from the Sierra Nevada mountains, which have high concentrations of arsenic. When the water arrives at Mono Lake, it has nowhere to go because there are no rivers carrying water farther downstream. That means the arsenic, and other elements and compounds, can concentrate to unusually high levels. After scooping up sediments containing a common microbe known as strain GFAJ-1 and taking itto the lab, researchers fed them a diet heavy on arsenic with just a little phosphorus. ASU associate research scientist Gwyneth Gordon made measurements to verify the diet being fed the microbes was high in arsenic and low in phosphorous. When the phosphorus was removed from their diet, the microbes kept growing. Analysis indicates the microbes use the arsenic to produce new cells.

Wolfe-Simon said she hoped to further test her findings in northern Argentina where, she's been told, some microbes can not only tolerate arsenic, but absolutely need arsenic to survive.

Arizona Republic reporter Anne Ryman and the Washington Post contributed to this article.